Naphthalenedicarboxylic acids are commercially important items as intermediates for chemical products, and in particular, have a widespread demand as starting materials for polyesters and polyamides that are used for fibers, bottles and films. Above all, 2,6-naphthalenedicarboxylic acid is useful as a starting material for polyethylene naphthalate (PEN) and aromatic liquid-crystal polymers having excellent physical characteristics and mechanical properties, and the demand for the acid is increasing these days.
As a method for producing a naphthalenedicarboxylic acid, there is known a method of oxidizing a naphthalene compound having two substituents with a molecular oxygen at high temperature and under high pressure in the presence of a heavy metal such as Co and Mn and a bromine compound in an acetic acid solvent. However, the product to be obtained according to the production method contains not only the intended naphthalenedicarboxylic acid but also, as an impurity, at least one selected from monocarboxylic acids and aldehydes that are intermediate products in oxidation reaction, catalysts-derived bromine adducts, and colored components and oxidation catalysts-derived metal components such as Co and Mn. The product that contains not only naphthalenedicarboxylic acid but also impurity is referred to as a crude naphthalenedicarboxylic acid.
When the crude naphthalenedicarboxylic acid is used as a starting material for polymer, the physical characteristics and the mechanical characteristics, such as heat resistance, mechanical strength, and dimensional stability, of the resin to be obtained are poor, and therefore the crude material is unsuitable as a starting material for polymer. In addition, the crude naphthalenedicarboxylic acid is generally colored in yellow, orange or black, and is therefore unsuitable for applications that require transparency such as bottles and films. Consequently, it is desired to increase the purity of naphthalenedicarboxylic acid by removing impurities, or that is, it is desired to develop an industrially advantageous production method for poorly-colored, high-purity naphthalenedicarboxylic acid.
In general, for purification of organic compounds, operation of distillation, crystallization, adsorption and the like is employed either singly or as combined. However, the self-decomposition temperature of naphthalenedicarboxylic acid is lower than the boiling point and the melting point thereof, and therefore the acid could not be purified through distillation. In addition, the acid is hardly soluble in various solvents, and is therefore difficult to purify through crystallization.
As in the above, the method of purifying the crude naphthalenedicarboxylic acid through conventional distillation or crystallization is difficult, and therefore a method of purifying the crude acid is proposed, which comprises reacting the naphthalenedicarboxylic acid with an amine to form an amine salt to thereby increase the solubility of the salt in water, alcohol or the like, then purifying the salt through crystallization or through treatment with active carbon, and thereafter decomposing the amine salt for purification of the acid.
PTL 1 discloses a method comprising dissolving a crude naphthalenedicarboxylic acid in an aqueous solution of an aliphatic amine, crystallizing the amine salt by cooling or condensation, and thereafter thermally decomposing the salt.
PTL 2 discloses a method for obtaining purified 2,6-naphthalenedicarboxylic acid, which comprises dissolving a crude 2,6-naphthalenedicarboxylic acid in an aqueous solution of an amine, then hydrogenating it and thereafter removing the amine compound through evaporation to thereby precipitate 2,6-naphthalenedicarboxylic acid.
PTL 3 discloses a method comprising crystallizing a naphthalenedicarboxylic acid amine salt with a water/ketone solvent and then decomposing the amine salt.
PTL 4 discloses a method comprising dissolving a salt of an aromatic dicarboxylic acid and an aliphatic amine and/or an alicyclic amine (hereinafter this may be referred to as an amine) in water, purifying it through adsorption to active carbon and thereafter heating it in the presence of water to decompose the amine salt.
PTL 5 discloses a method comprising blending a crude naphthalenedicarboxylic acid, an amine and a solvent containing water under the condition in which a crystal of a naphthalenedicarboxylic acid amine salt could precipitate out to give the crystal of the amine salt, and thereafter dissolving the amine salt crystal in water and then decomposing the salt.
PTL 6 discloses a method for producing a high-purity naphthalenedicarboxylic acid, which comprises dissolving a crude naphthalenedicarboxylic acid in an aqueous solution containing an aliphatic amine, then removing the heavy metal component through filtration or through filtration followed by solid adsorbent treatment in such a manner that the remaining heavy metal component could be at most 100 ppm relative to the naphthalenedicarboxylic acid, and thereafter heating the resulting aqueous solution to remove the amine through evaporation.
In PTL 6, a crude naphthalenedicarboxylic acid is dissolved in an aqueous amine solution and the metal component is precipitated out as an impurity, and thereafter the system is filtered through a filter having a pore size of at most 10 μm to separate the metal component. In addition, in this, there is given a description saying that in an industrial apparatus, it is better to gradually reduce the filter pore size in multistage filtration as preventing clogging and enabling long-term stable operation and that the heavy metal component hardly removable through filtration could be removed by additional solid adsorbent treatment.